The present invention relates to a radiation-curable heat-peelable pressure-sensitive adhesive sheet from which cut pieces of an adherend can be easily separated and recovered through irradiation with a radiation and a heat treatment. The invention further relates to a process for producing cut pieces with the adhesive sheet.
A heat-peelable pressure-sensitive adhesive sheet comprising a high modulus film or sheet substrate made of a plastic or the like and formed thereon a pressure-sensitive adhesive layer containing a blowing agent has been known as a pressure-sensitive adhesive sheet which is used in cutting a work to be cut, such as a semiconductor wafer or a multilayer capacitor sheet, into pieces of a given size in such a manner that the adhesive sheet is applied to the work (adherend) and the cut pieces, e.g., chips, are easily separated and recovered therefrom (see, for example, JP-B-50-13878 (the term xe2x80x9cJP-Bxe2x80x9d as used herein means an xe2x80x9cexamined Japanese patent publicationxe2x80x9d), JP-B-51-24534, JP-A-56-61468 (the term xe2x80x9cJP-Axe2x80x9d as used herein means an xe2x80x9cunexamined published Japanese patent applicationxe2x80x9d), JP-A-56-61469, and JP-A-60-252681). This heat-peelable pressure-sensitive adhesive sheet is intended to attain both the adhesive holding power which enables the adhesive sheet to withstand adherend cutting and the easy separation and recovery of resultant cut pieces therefrom. Namely, this pressure-sensitive adhesive sheet has the following feature. The adhesive sheet has high tackiness when in contact with an adherend. However, at the time when cut pieces are to be recovered, the foamable pressure-sensitive adhesive layer containing heat-expandable microspheres is foamed or expanded by heating to thereby come to have a roughened surface. Due to the resultant decrease in the area in which the adhesive layer is adherent to the adherend, the tackiness is reduced or lost and, hence, the cut pieces can be easily separated from the adhesive sheet.
However, the heat-peelable pressure-sensitive adhesive sheet described above has the following problems when used in cutting an adherend fixed thereto. Since the pressure-sensitive adhesive layer is soft and is thick because it contains heat-expandable microspheres, an adhesive waste is flung up by the cutting blade and the pressure-sensitive adhesive layer deforms to cause chipping. An effective measure in overcoming these problems is to reduce the thickness of the pressure-sensitive adhesive layer. However, if the heat-peelable pressure-sensitive adhesive sheet described above is produced so that the pressure-sensitive adhesive layer has a reduced thickness not larger than the size of the heat-expandable microspheres, then the heat-expandable microspheres partly protrude from the surface of the pressure-sensitive adhesive layer to impair the surface smoothness of the pressure-sensitive adhesive layer. This pressure-sensitive adhesive, sheet cannot have tackiness sufficient to hold an adherend thereon. Consequently, thickness reduction in the pressure-sensitive adhesive layer is limited and there are cases where those problems remain unsolved.
On the other hand, a radiation-curable pressure-sensitive adhesive sheet also is extensively used in cutting a work into pieces in such a manner that the adhesive sheet is applied to the work and the cut pieces, e.g., chips, are separated and recovered therefrom. The radiation-curable pressure-sensitive adhesive sheet generally has a pressure-sensitive adhesive layer containing a radiation-curable compound, and is characterized in that when the cut pieces resulting from the cutting are to be recovered, the radiation-curable pressure-sensitive adhesive sheet is irradiated with a radiation to cure the pressure-sensitive adhesive layer and thereby considerably reduce the tackiness thereof. In this radiation-curable pressure-sensitive adhesive sheet, the thickness of the pressure-sensitive adhesive layer can be reduced infinitesimally and this is advantageous in preventing the adhesive from being flung up or deformed. However, use of the conventional radiation-curable pressure-sensitive adhesive sheet has the following problem. Since the pressure-sensitive adhesive layer, even after having been cured by irradiation with a radiation, generally has residual tackiness, the recovery of cut pieces necessitates a pickup operation in which a physical stress is necessary for, e.g., pushing up the cut pieces. It has been pointed out that this pickup recovery operation may mar or crack the cut pieces when the cut pieces are exceedingly thin like semiconductor wafers and the like, whose thickness is decreasing recently.
In JP-B-63-17981 is disclosed a pressure-sensitive adhesive sheet for use in cutting a semiconductor wafer into pieces. This pressure-sensitive adhesive sheet comprises a substrate coated with a pressure-sensitive adhesive layer comprising a pressure-sensitive adhesive, a radiation-polymerizable compound, and a heat-expandable compound. In the patent document cited above is also disclosed a method which comprises adhering a semiconductor wafer to the surface of the pressure-sensitive adhesive layer of the pressure-sensitive adhesive sheet, dicing the wafer, subsequently conducting irradiation with a radiation and a thermal expansion treatment simultaneously to reduce the tackiness, and then picking up the cut pieces. This method, however, has the following drawback. Since the irradiation with a radiation and thermal expansion treatment for reducing tackiness are simultaneously conducted after the cutting (dicing) step, an adhesive waste is flung up and the adhesive layer deforms during the work-cutting operation. As a result, the precision of cutting is low. In addition, since the pressure-sensitive adhesive layer has high residual tackiness after the irradiation with a radiation and thermal expansion treatment, it is difficult to recover exceedingly thin cut pieces.
One object of the, invention is to provide a radiation-curable heat-peelable pressure-sensitive adhesive sheet which has tackiness enabling adherends to withstand transportation and other steps, with which cutting can be conducted without flinging up an adhesive waste or causing chipping, and from which the cut pieces can be easily separated and recovered.
Another object of the invention is to provide a process for producing cut pieces with the pressure-sensitive adhesive sheet.
As a result of intensive studies to accomplish those objects, it has been found the following.
A pressure-sensitive adhesive sheet comprising a substrate and formed on a surface thereof a pressure-sensitive adhesive layer having both radiation curability and thermal expansibility can be used to cut a work and efficiently and smoothly separate and recover the cut pieces therefrom without arousing troubles in processing steps, when the thermal expansibility is imparted with heat-expandable microspheres and the work-cutting step is conducted after the curing of the pressure-sensitive adhesive layer by irradiation with a radiation and before the thermal foaming of the pressure-sensitive adhesive layer. The invention has been completed based on this finding.
The invention provides a radiation-curable heat-peelable pressure-sensitive adhesive sheet which comprises a substrate and, formed on at least one side thereof, a pressure-sensitive adhesive layer containing heat-expandable microspheres and a radiation-curable compound.
The invention further provides a process for producing cut pieces which comprises placing a work to be cut on the surface of the pressure-sensitive adhesive layer of the radiation-curable heat-peelable pressure-sensitive adhesive sheet described above, irradiating the pressure-sensitive adhesive layer with a radiation to cure the adhesive layer, cutting the work into pieces, subsequently thermally foaming the pressure-sensitive adhesive layer, and then separating and recovering the cut pieces from the adhesive sheet.